Ooops - hit "submit" too soon.... (it's an age thing)
C4s were not available in 69s, but C6s were - with a big block. So if you have the original and a 351 then chances are very good that you have an FMX.
Crawl under there and look at it.
An FMX is in 3 pieces - bell housing, body, tail shaft. The middle section is iron(?) - it's not aluminum like the bell and tail.
A C6 is 2 pieces, both aluminum.